The central feature of the extremely rare multiple-personality 
disorder is the existence of two or more apparently distinct 
personalities in one individual. Each of the personalities is 
associated with unique ways of thinking, behaving, and expressing 
emotion. In some cases the multiple personalities are fully 
developed in the sense that they are associated with distinctly 
different types of identities and have different memories for past 
events. In other cases, characteristics of the personalities 
overlap so that they share some traits and memories. Either way, 
a person with the disorder responds differently on psychological 
tests, and may even demonstrate different patterns of brain 
activity, for each personality. The transition from one 
personality to another typically is abrupt and is precipitated by 
a stressful experience or environmental cue. 

Patients with multiple-personality disorder vary in the extent 
to which they are aware of their condition. Some are unaware that 
they manifest dramatic changes in personality, whereas others are 
conscious of the fact that they show distinct personalities. The 
extent of the functional impairment in these patients varies from 
mild to severe. The cause of the disorder is as yet unknown. 
PSYCHOTHERAPY is the most common form of treatment for multiple-
personality disorder. 

Studies of multiple-personality disorder report that it occurs 
from three to nine times more frequently in females than in males. 
Onset of the disorder often occurs in childhood, and in almost all 
cases the disorder is preceded by some form of abuse or other 
traumatic event. Confirmed cases of multiple-personality disorder 
number only in the hundreds. Recent research has suggested, 
however, that multiple-personality disorder may occur more often 
than previously thought. 

Bibliography: Bootzin, R. A., and Acocella, J. R., Abnormal 
Psychology, 5th ed. (1988); Confer, W. N., and Ables, B. S., 
Multiple Personality (1983); Endler, N. S., and Hunt, J. M., 
Personality and the Behavior Disorders, 2 vols., rev. ed. (1984).